Molecular simulation of enhanced CH4 recovery and CO2 storage by CO2-N2 mixture injection in deformable organic micropores

The injection of CO2-N-2 mixture for enhanced gas recovery is a potential technique to improve CH4 production and reduce formation swelling for shale and coal. Understanding the displacement process of the pre-adsorbed CH4 by the injected gas in the organic pores is essential for the production of shale gas and coalbed methane. In this work, we have used the deformable organic slit pore model to study the CH4 displacement by CH4-N2 mixture with various compositions with grand canonical Monte Carlo simulation. The CH4 recovery, CO2 storage and corresponding pore deformation were investigated under geological conditions. It was found that the injection of CO2-N-2 mixture reduced formation swelling compared to pure CO2 injection, and meanwhile achieved high CH4 recovery and CO2 storage. But the efficiency depended on the injected gas composition and pore size. The CH4 recovery increased with the increase of total CO2-N-2 mixture fraction despite different CO2/N-2 ratios. The CH4 recovery, CO2 adsorption and deformation generally decreased with the increase of pore size, and become relatively constant for the pores above 1.1 nm. Adding N-2 in the gas injection effectively reduced the swelling in the 0.75-0.95 nm and 1.1-2.0 nm pores. The molecular analysis showed that CO2 and N-2 can individually induce swelling or shrinkage. The adsorbed N-2 caused little deformation, but affected the deformation through CH4 desorption. The enhanced gas recovery was studied aiming at geological conditions with various pressures and temperatures, and it was found that the CH4 recovery with the CO2-N-2 mixture injection was improved at all depths. The recovered CH4 and CO2 storage increased significantly with depth to 200 m, and reached the maximum at formation deeper than 500 m and 200-500 m respectively depending on formation pore size distribution.